Spring-applied, pressure-released brakes have certain inherent advantages, such as automatic application upon loss of release pressure. The usefulness of such brakes however, has been hindered by the difficulty in providing a means for controlling the application of the brakes, i.e. so that the brakes are applied smoothly and predictably. On some vehicles, such as construction equipment, which are commonly provided with hydraulically actuated tools, the most convenient medium for release of the brakes is the readily available hydraulic pressure. However, the viscosity of hydraulic fluid varies with temperature, further complicating the control of such brakes when utilized on vehicles of this type. Also certain types of construction equipment are provided with articulated frames for steering which introduces still further difficulties, particularly when the pressure source and the operator compartment are located on different frame sections. The reason for this difficulty is due to the need for rapid evacuation of the brake cylinders in order to achieve quick brake response. This requirement dictates the location of the brake valve in close physical proximity to the hydraulic pump and reservoir, while the brake pedal obviously must be located in the operator compartment which often is on the other frame section.
It is, therefore, an object of this invention to provide a means for controlling a spring-apply, pressure-release brake system which permits a smooth and predictable application of the brake and still has a normal "feel" for an operator thereof.
It is also an object to provide such a control means which utilizes hydraulic fluid as the pressure medium and in which the temperature effects are minimized.
It is another object to provide such a control means which permits a quick response and yet is utilizable on an articulated vehicle without the need for complicated mechanicial connections between the operator compartment and the brake valve.
It is still another object to provide such a control means which permits towing of the vehicle when the engine thereof is not operating or release pressure normally available is otherwise lost.
It is also an object to provide such a control means which permits positive application of the spring-applied brakes for maintenance and repair, and which precludes accidential or inadvertent release thereof.
It is also an object to provide such a control means which will permit application of at least a portion of the brake system even through another portion of the system may have failed.
These and other objects of the present invention, and many of the attendant advantages thereof, will become more readily apparent from a persual of the following description in connection with the accompanying drawings wherein:
FIG. 1A and 1B, combined, represent a hydraulic schematic of a preferred embodiment of the invention as applied to an articulated loader.